Black holes – why do galaxies care, anyway?

Now that our paper on AGN host galaxies (galaxies whose black holes are feeding) is out, I will write a few blog posts about what we found with your help. But before we start, a little background.

Why do black holes matter? We now believe that at the centers of most, if not all galaxies, there is a supermassive black hole. We call these black holes “supermassive” to distinguish them from stellar mass black holes that were formed in the deaths of massive stars. These supermassive black hole can be as heavy as a million or even a billion solar masses.

So you might think that these enormous black holes can wreak havoc in their host galaxies. However, galaxies are even bigger, much bigger than these black holes. In general, the black hole makes up about 0.1% of the mass of its host galaxy making really just a drop in the bucket.

In fact, their gravitational sphere of influence is tiny compared to the size of the whole galaxy and so they generally don’t affect anything but their immediate surroundings. As far as the galaxy as a whole is concerned, the supermassive black hole at its center might as well not be there.

But why is the mass of the black hole always some fraction of the galaxy mass (or to be more precise, bulge mass)? How does the black hole even know how big the galaxy is? Why does the mass of the black hole correlate with the mass of the galaxy bulge (the M-sigma relation)? It’s almost as if they somehow grew together….

Could it be that an original stellar mass black hole formed from early massive stars before the rest of the accreting galaxy-to-be and gravitationally grew bigger on infalling gas and then nearby stars? Would this ultimately impart a rotation to the growing mass of the hole and then the accreting proto-galaxy itself, simply under gravity?